The invention relates to a bone fixation system and method and, more particularly, to a cable anchoring apparatus for using cables to stabilize bones relative to each other.
It is known to use cables anchored to bones for various medical procedures. For example, with respect to the spinal column, several applications are apparent. However, one significant limitation with these applications is the need for posterior procedures, particularly where anterior procedures would be preferable, but whose performance is limited by the space required of current cable anchors in the body cavity. Anterior along the spinal column are significant impediments to having cable anchor members projecting from the vertebrae in which they are sunk such as surrounding viscera including organs, intestines and large blood vessel groupings. Another problem is the toggling effect projecting anchors can create with tensioned cables extending transverse thereto and thus generating a bending movement at the projecting anchor head.
Accordingly, there is a need for an improved cable bone fixation system, and particularly a system and method that allows for cables to be anchored anteriorly along the spinal column with potential for interference with surrounding viscera minimized.
In accordance with the present invention, a system and method for maintaining positions of bones fixed or approximated relative to each other is provided that is ideal for minimizing interference with surrounding viscera in spinal column procedures, although it will be recognized its use is not so limited as the robust and secure anchoring provided thereby will be desirable in many bone fixation procedures. In the preferred form, a cable anchoring apparatus in the form of a screw member having an elongate shank that is threaded for substantially the full length thereof is employed. An internal driver surface is provided so that the size of the proximate end of the shank can be minimized; in other words, the shank proximate end is maintained consistently sized with respect to the reminder of the shank, i.e. no enlarged driver head is formed thereat. This allows the amount of bone material that is removed from full insertion of the screw anchor to be minimized, i.e. no countersinking for an enlarged driver head is necessary, thus improving holding power of the cable anchor herein. Further, the full threading of the shank for substantially its entire length enables the screw member to be fully sunk into the bone so that no portions thereof, such as an enlarged screw head, project into the surrounding body cavity in which the bone is located.
A flexible cable is attached to the screw to be anchored to the bone and for being attached to another anchored cable so that surfaces of bones or bone portions having the cables anchored thereto can be approximated in fixed position relative to each other. Also, with the screw anchors fully sunk into the bones, the cables are able to ride on the bone surfaces to provide them with a large bearing surface along their length so as to minimize points of stress concentration therealong and the potential for wear these create.
Herein, it will be understood that the terms bones or bone portions are interchangeable and can refer to distinct bones such as vertebrae in a spinal column or portions of a single vertebrae bone or other bone. Generally, in the single bone aspect, the surfaces to be approximated are those at the fracture, whereas with distinct bones, it is the facing surfaces of the bones which are the surfaces that are desired to be held in substantially fixed positions relative to each other despite dynamic motion of the body part, e.g. spinal column, that they support.
In terms of procedures or indications in which the cable anchor apparatus herein can be used, one typical procedure is in conjunction with intervertebral decompression devices such as an adjunct to a cage or barrel used to support adjacent vertebrae in a fixed, spaced position relative to each other. The preferred spinal levels of use are L5 (fifth lumbar vertebra)xe2x80x94S1 (adjacent sacrum bone) and L4 (fourth lumbar vertebra)xe2x80x94L5. In this application, two screw anchors can be inserted into each bone portion with the cables thereof interconnected by connectors such as crimp connectors so that the cables form a criss-cross cable pattern as they extend across the gap between the two vertebrae with the cage or cages therebetween. The cris-cross cable pattern provides increases resistance against the torsional forces that the spinal column generally creates during dynamic motion thereof so as to keep the facing vertebrae surfaces approximated and minimizes motion of these surfaces that can be detrimental to proper healing and the healing process itself. In this regard, the cross-cables also tend to minimize the tendency for the decompression cages to back out of the space between the vertebrae with motion of the spinal column.
The cable anchoring system herein can also be used laterally on the spine such as for reducing scoliosis. Depending on the curvature of the spine to be corrected, the cable anchors are applied into two adjacent vertebrae with the cables thereof tensioned and crimped together via the crimp connector to apply a counteracting force against the curvature toward a straightening of the spinal column. In a pubis symphysis fracture, the cable screw anchor is inserted into each pubis, and the cables are then connected. In addition to anterior spinal stabilization such as with the above-described multiple pairs of cable screw anchors having their associated cables connected and tensioned in a criss-cross fashion, lateral spinal stabilization can also be accomplished with the present cable screw anchors. The cable screw anchors can also be used for posterior spinal stabilization such as to secure a laminar fracture. Each cable screw anchor is screwed into the pedicle on each side of a spinous process. The tensioned cables pull the fractured part of the vertebra into position for proper healing to occur.